1. Field of the Invention
This invention relates to a pneumatic radial tire for heavy load vehicles, particularly trucks and buses which can effectively prevent wandering phenomenon.
2. Description of the Prior Art
The wandering phenomenon shall be understood to mean handle movement away from the usual route which has often been encountered by heavy load vehicles such as a large type truck or the like. This occurs when the vehicles run in a wheel print produced by dipping, particularly by the wearing of a paved road surface by the weight of vehicles which run frequently thereon and when the vehicles tend to getaway from the wheel print for the purpose of changing the route or of avoiding obstacles scattered on the road surface.
The wheel print produced on the paved road is primarily about 2 cm in depth and eventually inclusive of a deep wheel print which is deeper than 3 cm. The wandering phenomenon seldom occurs for passenger tires inflated by an internal pressure which is considerably lower than that used for the radial tire for heavy load vehicles and for bias-laid tire for heavy-load vehicles.
The wandering phenomenon exerts significant influences upon the radial tire for heavy load vehicles including a tread significantly reinforced by a belt having an extremely high rigidity and incorporated into a crown portion and inflated at a high internal pressure, that is, upon the radial tires for trucks and buses. The wandering phenomenon is rather inherent to these tires.
The radial tires for trucks and buses are of a so-called square shoulder in which the boundary between the tread and the side wall, that is, the shoulder is composed of a square ridge. In order to alleviate resistance to the tire produced when it rides on the wheel print, the square shoulder is made round to provide a so-called round shoulder, thereby preventing the wandering phenomenon.
But, the deflection of the round shoulder tire is under the influence of the amount of load, dynamic load due to oscillations of the tire when it runs, and as a result, the round shoulder partly functions as a ground contact region. Hence, the tire tends to be easily subjected to abnormal wear unless the internal pressure of the tire is successfully controlled. That is, this partial ground contact region is composed of a circle-shaped portion, so that the ground contact pressure is smaller than that of the flat tread. As a result, the partial ground contact region tends to easily respond to the local slide or movement thereof with respect to the road surface, thereby causing shoulder wear which is an abnormal premature wear induced at both side edges of the tread.